Method and device for preparing gravel

ABSTRACT

In processing of raw gravel to remove lump materials of loam or the like in which a processing step such as screening or washing of the gravel is employed, the raw materials are mechanically comminuted before the processing step, the comminution step comprising stirring the raw gravel with a stirrer to break up the extraneous lumps. An apparatus for comminuting the raw gravel has a vertical tubular container. The raw gravel is introduced to the top of the container, and removed at the bottom of the container on a container trough. A stirrer is provided within the container for stirring the raw gravel as it descends in the container, and means are provided for controlling the speed of downward movement of the gravel.

@mted States Patent 1191 [111 3,845,905

Haslherger Nov. 5, 1974 METHOD AND DEVICE FOR PREPARING 1,636,033 7/1927 Agnew 241/188 GRAVEL 1,725,868 8/1929 Kenyon 241/188 A X 1,997,031 4/1935 Allswede 24l/26 X Inventor: Josef Haslberger, Landshuter 3,329,350 7 1967 Wisgerhofet al. 241/188 R x Strasse 30, D 805 Freising, 3,612,420 10/1971 Hull 241/197 Germany [22] Filed: Mar. 5, 1973 Primary Examiner-Granville Y. Custer, Jr.

[21] App]. No.: 337,959 57 ABSTRACT In processing of raw gravel to remove lump materials [30] Foreign Application Priority Data of loam or the like in which a processing step such as Mar. 14, 1972 Germany 2212251 Screening Of washing of the gravel is employed, the raw materials are mechanically comminuted before 52 us. c1 .241/26, 241/27, 241/188 R, the processing Step, the comminution p comprising 241/1 1 stirring the raw gravel with a stirrer to break up the 51 Int. Cl. B026 17/16 extraneous p An apparatus for comminuting h 53] Field f Search 241 2 7 186 R, 1861, raw gravel has a vertical tubular container. The raw 241 13 2 33 R 245 24 249 25 R, gravel is introduced to the top of the container, and 2 22 300 01 9 101 B 1 5 B 2 4; removed at the bottom of the container on a container 259/ 27 24 44 10 trough. A stirrer is provided within the container for stirring the raw gravel as it descends in the container,. [5 R f r n Cited and means are-provided for controlling the speed of downward movement Of the gravel.

768,955 8 1904 Russell 241 /188R x 12 Claims, 4 Drawing Figures sum 10F 3 PATENTED BUY 5 I974 PAIENTEUnnv 51914 Sis-45905 I SHEET 30F 3.

FIG. 4

METHOD AND DEVICE FOR PREPARING GRAVEL The invention relates to a method and an apparatus for preparing raw gravel, which in addition to gravel itself or gravel stones also contains extraneous lump materials such as loam, marl or the like, and the extraneous materials are removed by screening and/or washing of the gravel.

When the raw gravel is obtained, for example in gravel pits, and contains large amounts of loam, marl or the like, the rawgravel is unsuitable for many purposes. lt is therefore desired to free the raw gravel of the extraneous materials, and more particularly to clean it. For this purpose attempts have already been made to remove the extraneous materials from the raw gravel by screening or sieving. In this case, however, only such extraneous materials can be removed from the raw gravel as have a diameter different from that of the gravel stones or particles. If, for example, from the raw gravel particles are screened in the size range 3/50 (diameter between 3 and 50 mm) or 3/80, the screened raw gravel will still contain extraneous materials lumps with diameters between 3 and 50 mm and between 3 and 80 mm respectively. However, these extraneous materials in lump'form are also to be removed.

In accordance with a further prior proposal raw gravel was to be cleaned by washing. ln this case the extraneous materials such as loam, marl or the like are broken up in the water and swilled away with the water from the gravel. However, this washing treatment can only be used to achieve a certain degree of success in the case of smaller size ranges, for example /3, while in the case of raw gravel with larger particles, for example 3/50. the washing process lasts too long and in the case of a comparatively long washing treatment there is only a partial breaking up and swilling away of the extraneous materials.

In contrast to this the aim of the present invention is to prepare the raw gravel rapidly and completely, that is to say free it from extraneous materials such as loam, marl or the like.

In accordance with the invention the method is so carried out that the extraneous lump materials are mechanically comminuted before screening and/or washing. The mechanical comminution is preferably carried out by treating the raw gravel mechanically so that in the course of this treatment the extraneous lump materials are crushed by the gravel stones. In the case of the method in accordance with the invention advantage is particularly taken of the fact that the gravel and the extraneous materials have a differing degree of mechanical hardness so that the gravel serves as a means for the mechanical impacting of the raw gravel mixture in order to crush the lumps of extraneous material.

Although mechanical comminution of extraneous material in lump form can be carried out in various different manners, for example by stamping, rolling or crushing of the raw gravel in a moist or a dry condition or by using centrifugal forces, in accordance with a preferred embodiment of the method the raw gravel is stirred in a container with a stirring means and the container maintains the raw gravel in the zone of action of the stirrer so that the inner friction between the gravel stones and the extraneous materials ensures that the lumps of extraneous material are crushed. In the case of this procedure there is a particularly high efficifency and the inner friction is effectively made use 0 Preferably the raw gravel is moved by the stirrer and- /or the action of gravity through a tube-like container in a continuous manner. This continuous treatment has the advantage over working in batches that the method can be better adapted in the whole course of the process in the case of the preparation of the gravel.

Preferably the speed of filling and the speed of discharge for the container are so controlled that the container is always tilled with raw gravel in operation. Owing to the fact that the container is always filled, it is possible to ensure that the stirrer always operates with a maximum efficiency.

Finally, a preferred embodiment of the method in accordance with the invention resides in that the raw gravel with a size of greater than approximately 3 mm or in the particle size range of for example 3/50; 3/80 or 5/100 is treated. Owing to the fact that material in the particle size range of 0/3 mm is separated off before the raw gravel is prepared in accordance with the method of the invention, the apparatus respectively provided for carrying out the method in accordance with the invention is not loaded with such components of the raw gravel, which can be cleaned with a certain degree of efficiency even without comminution of the extraneous materials, for example by washing alone. The upper limit of the particle size range of the material to be processed or generally the treatment of raw gravel within certain particle size limits offers the advantage that the dimensions and the throughput design of the apparatus provided for carrying out the method in accordance with the invention can be selected more judiciously.

In the case of the method in accordance with the invention it is more particularly advantageous for the lumplike extraneous materials to be comminuted in the shortest possible time to such an extent that they can then also be screened out in a short time, though in this case washing is in fact preferable to screening. In this manner it is possible to remove rapidly and completely lumps of extraneous material with a diameter of approximately 50 mm. Such lumps cannot be removed by washing because they require such a long period of time for softening that the whole preparation treatment would be delayed to such an extent that it would no longer be economic.

The invention also relates to an apparatus for carrying out the above-described method, comprising a screening and/or washing device for removing extraneous materials from the raw gravel and is characterised by a tubular container having at least one charging opening and at least one discharging opening for the raw gravel, and in the container a stirrer'is arranged. In the case of this device the container has the purpose of keeping the raw gravel in the range of action of the stirrer. The stirrer brings about an intimate mixing or working of the raw gravel and owing to the inner friction the soft extraneous material in lump form is ground by the hard stones of the gravel. The stirrer pushes the raw gravel in front of it and thus creates a build-up zone in which the raw gravel is pressed together so that the stones in the raw gravel crush the lumps of extraneous material. When the raw gravel which has been built up or piled up leaves the build up zone, it is loosened somewhat again. Then it passes again into a pile up zone, where it is again pressed together. Owing to this process which is repeated for each rotation of the stirrer a rapid and complete breaking or grinding up of the soft lumps of extraneous material is ensured.

Preferably the tubular container is closed at its charging and its discharging ends by end walls and the openings for charging and discharging the raw gravel are provided in the end walls. Owing to this construction of the container advantageous continuous operation is made possible.

In accordance with the shape of the stirrer the latter also brings about the transport of the raw gravel through the container. The movement of the raw gravel through the container can, however, in accordance with a preferred form of the apparatus in accordance with the invention, be reinforced or taken over by arranging the container with its longitudinal axis at an angle to the horizontal, and preferably perpendicular to it and by adopting the feature that the charging opening or the charging openings are arranged at the upper end of the container and the discharge opening or the discharge openings are arranged at the bottom end of the container.

At each charging opening a filling hopper is preferably provided for the raw gravel and in the hopper, in accordance with a further embodiment of the invention, means are provided for controlling the speed of the discharge of raw gravel from the hoppers. In this manner the speed of filling or charging can be so controlled that the container is always filled.

Downstream from the discharge opening or the discharge openings a jog conveyor trough is preferably provided for removing the raw gravel. The conveyor trough can be actuated by a variable speed jog motor. This provides a further means for controlling the speed of passage of the raw gravel through the container. Thus, if the conveyor trough removes the raw gravel mores slowly than it can be discharged from the container, a cone of discharge material is formed at the discharge opening of the container so that the raw gravel is dammed in the container.

Preferably the stirrer has several stirring arms arranged on a driven shaft. Front faces, arranged in front in the direction of movement of the stirrer, of the stirring arms can have an angle of attack such that the front faces are slightly directed in the direction of passage of the raw gravel so that transport of the raw gravel is encouraged. In order to increase the abovementioned pile up effect of the stirrer the front faces, lying to the front in the direction of movement of the stirrer, of the stirring arms are, however, preferably so arranged that in the case of a section through the stirring arms in a plane parallel to the longitudinal axis of the shaft there is a section line, parallel to the longitudinal axis of the shaft, at the respective front face. The front faces, in other words, have no inclination with respect to the conveying device, lying parallel to the Iongitudinal axis of the shaft, for the raw gravel. In this case gravity is sufficient for transport of the raw gravel.

The speed of rotation of the shaft, which carries the stirring arms. can preferably be varied. In this manner it is possible to adjust the speed of rotation of the shaft to various sizes of the raw gravel to be processed. Furthermore, there is also the provision of a further possibility of controlling the speed of flow of raw gravel through the container.

In accordance with a further form of the invention the stirring arms each comprise a support and a wearprone part, which lies in the direction of movement of the arm on the front side of the support. Accordingly after wear it is a simple and quick matter to bring the apparatus in accordance with the invention back into its functional condition, that is to say to repair it. The stirring arms can be arranged in pairs at the same level of the shaft. If two stirring arms of a pair are aligned with each other it is advantageous for the stirring arms of two stirring arm pairs lying adjacent to each other to be offset by a constant angle, preferably about the shaft. This results in a more even loading of the shaft in operation.

On the inner wall of the container it is possible to provide sheet metal guides which force the raw gravel to pass through the range of action of the stirrer. Preferably the sheet metal guides are in the form of annular discs, extending inwards from the inner wall of the container, and whose internal diameter is smaller than the range of operation of the stirrer. The sheet metal guides ensure that the raw gravel cannot move along the inner wall of the container without being engaged by the stirrer and worked by it.

The apparatus in accordance with the invention is particularly suitable for incorporation in a continuously operating processing or preparing installation for raw gravel. An installation, in which the apparatus in accordance with the invention is provided,'can then have the following stations in chronological order: a supply station for the raw gravel, a sieve or screen, which divides the raw gravel into various particle size ranges, the apparatus in accordance with the invention for the comminution of extraneous lumpy materials, a sieve or screen for screening out the extraneous materials from the gravel after they have been comminuted, the size of the gravel having remained the same, and/or a washing device, in which the comminuted extraneous materials are swilled or washed out. The action of the apparatus in accordance with the invention in such an installation is reflected in a substantial increase in the throughput (quantity of gravel per unit time) and a higher degree of purity of the gravel, which is removed from the last screen or from the washing device arranged at the end of the installation. A further advantage of the apparatus in accordance with the invention is to be found in that the possiblity of controlling the speed of discharge of the raw grave], the possibility of controlling the speed of rotation of the stirrer and the possibility of controlling the conveying speed of removal on the conveying trough lead to sufficient possibilities for bringing the apparatus in accordance with the invention into agreement with the throughput speed of the remainder of the installation and the various ranges of particle size in the raw gravel to be processed.

Further advantageous forms of the invention are to be found in the remaining subclaims. An embodiment of the apparatus in accordance with the invention is shown in the accompanying drawings.

FIG. 1 is a vertical section through the apparatus in accordance with the invention which in essence is shown schematically.

FIG. 2 is a plan view from above of the apparatus in accordance with the invention, the upper end wall of the container being omitted.

FIG. 3 is a. plan view looking onto a horizontal section through the apparatus in accordance with the invention on the line 3-3 of FIG. 1.

FIG. 4 is a section through a stirring arm on the line 4-4 of FIG. 3. I

In FIG. 1 the main part of the apparatus in accordance with the invention is shown schematically, in which the extraneous lump materials such as loam, marl or the like are comminuted. The sieving (or screening) or the washing device, in which the comminuted extraneous materials are then removed from the raw gravel, is constructed in a conventional manner and has therefore not been shown for purposes of simplification.

The device shown in FIG. 1 for comminuting the extraneous materials comprises a tubular container with two charging openings 12, 12' and two discharge openings l4, 14. The charging openings l2, l2 and the discharge openings 14, 14' are'placed in the end walls 16 and 18 respectively, which terminate the container I0 above and below respectively. The longitudinalaxis of the container is vertical. Thus, the charging openings 12, 12' lie above the discharge openings l4, 14' so that in the case of this container continuous operation with a gravity conveying action is possible. The charging openings-l2, l2 and the discharge openings 14, l4'- lie respectively outside the longitudinal center axis of the container 10 so that in the upper end wall 16 and the lower end wall 18 in the middle there is sufficient room for'further parts of the apparatus, as will be described below.

At each charging opening 12, 12' a filling hopper 20, 20' is provided, through which the raw gravel is charged into the container 10. In the hoppers 20, 20'

devices are provided for controlling the speed with 28 respectively via shafts. The motors 26, 26' and 28,

28 are preferably of a variable speed construction. The control of the discharge speed of the raw gravel from the hoppers 20, 20 can, however, be carried out by switching on and off some or all of the motors 26, 26' and 28, 28. Owing'to the spatial arrangement of the stirring vanes 22,22 and 24, 24' it is possible to keep the whole volume of the hopper 20 or of the hopper 20' under the influence of the control means during flow. This ensures reliably that the raw gravel, flowing out of the lower part of the hopper. does not move out of the upper part of the same hopper as an irregular follow-up pace. for example because it has piled up or become jammed.

At the discharge openings l4, l4 tubular spouts 30, 30' are provided, under which jog conveyor troughs 32, 32' are provided. which are driven by motors 34, 34'. The motors 34, 34 are also of variable speed construction so that various speeds of conveying on the jog conveyor troughs 32, 32' can be selected. This makes possible a control of the discharge speed of the raw gravel discharged from the container 10.

The possibility of controlling the discharge of raw gravel from the container 10 will be understood on making the following consideration. When the motors 34, 34' of the conveyor troughs 32, 32' are stopped, the raw gravel will pile up in cones or heaps beneath the spouts 30, 30 and the conveyor troughs 32, 32 so that no further discharge of raw gravel can occur. If, on the other hand, the conveyor troughs 32, 32 remove the raw gravel more rapidly than it is supplied from the discharge openings 14, 14, the container 10 will be empty after a certain elapse of time, something which is also undesirable. Between these two extremes the speed of discharge of the raw gravel through the container 10 can be controlled. The motors 26, 26' and 28, 28' and the motors 34, 34 can be connected together in a simple control circuit with a limit switch (not shown) and the circuit ensures that the container 10 is always completely filled with gravel in operation.

In the container 10 a stirrer 40 is provided, which extends as far as a position adjacent to the inner wall of the container. This ensures that the whole of the volume of raw gravel contained in the container 10 is engaged by the stirrer 40. The stirrer 40 has several stirring arms 42, which, are fixed on a shaft 44. In all 14 stirring arms. are present. The shaft 44 is journalled in the middle of the upper end wall 16 and in the middle of the lower end wall 18 of the container 10 by means of bearings 46 and 48, which are preferably ball or roller bearings-The shaft 44 extends through the bearing 46 above the upper end wall 16 into a transmission 50, which is driven by a motor 52 (FIG. 2). The motor 52 runs with a constant speed. The transmission 50 makes it possible, however, to vary or control the speedof rotation of the shaft. By varying the speed of rotation of the shaft 44 it is possible to take into account the influence of various particle size ranges in the raw gravel. Furthermore, an influence on the speed through the container 10 of the raw gravel can be exerted. In certain circumstances the control of the transmission can also be carried out via the control device mentioned above in connection with the motors 26, 26; 28, 28'; 34, 34.

Between the bearings 46 and 48 the shaft 44 has a square cross-section so that the stirring arms can be attached to the shaft 44 in a particularly simple manner which prevents any independent rotation. The stirring arms 42 extend radially from the shaft 44 in an outer direction and end at positions adjacent to the inner wall of the container 10. Each two stirring arms 42 are attached so as to be in mutual alignment at the same level of the shaft 44. The arms 42 of two adjacently placed pairs of arms are offset by about the shaft 44. This can be seen from FIG. 1 in which the first, third, fifth and seventh pair of arms is vertical to the plane of the drawing while the second, fourth andsixth pair of arms lie substantially in the plane of the drawing.

Approximately halfway between the arm pairs annular discs 54 are provided serving as sheet metal guides. They extend inwardly from the inner wall of the container in order to force the raw gravel to pass through the path of action of the stirrer 44. For this purpose the inner diameter of the annular discs 54 is smaller than the path of action of the stirrer 40. In some instances, however, it may be sufficient if the'inner diameter of the annular discs is the same in size as the outer radius of the rotating movement of the stirring arms 42. The sheet metal guides are intended to prevent the raw gravel being able to move along the inner wall of the container 10 without being engaged by the stirrer 40.

An embodiment of the stirring arms 42 is shown in FIG. 4. The stirring arms 42 have a rectangular support 60 and wear-prone part 62 attached to it. The wearprone part 62 is arranged on the front side of the support 60 as seen in the direction of movement of the arm. The wear-prone part 62 is fixed by means of screws 64 to the support 60. The screws 64 extend, in accordance with the embodiment of FIG. 4, from the support 60 into the wear-prone part 62,'and the screw heads are sunk in the support 60. The screw arrangement shown has the advantage that in the case of wear of the wear-prone part 62 occurring the head of the screw 64 is not also subject to wear, because it is arranged on the protective side in the support..On undoing the screws for removing wear-prone parts after wear an intact screw head will in every case be present, something which substantially facilitates operation.

The wear-prone part 62 is to cover at least the front side of the support 60. In the case of the embodiment shown in FIG. 4 the wear-prone part 62 is constructed in the form of a letter U, and the support 60 has a rectangular form which fits into the U-shape of the wearprone part 62. In this manner not only is the front face of the support 60 but also its side faces partly protected.

The chief purpose of the stirrer 40 is to work the raw gravel contained in the container 10 owing to inner friction between the gravel stones and the extraneous lumpy materials the extraneous materials are comminuted. It would also be possible to use the stirrer 40 to convey the raw gravel through the container 10. For this purpose it would only be necessary to arrange the front faces of the stirring arms at an angle to the vertical perpendicular so that these faces would be directed towards the discharge end of the container 10. In the case of the embodiment shown this measure is not adopted because the conveying action of gravity together with the pure stirring action of the stirrer 40 have been found to be sufficient in order to move the raw gravel through the container 10. In the case of the embodiment shown, therefore, the front faces, that is to say the faces in the front considering the direction of movement of the stirrer 40, of the stirring arms 42 are so arranged that in the case of a section through the stirring arms 42 in a plane parallel to the longitudinal axis of the shaft there is a line of section, parallel to the longitudinal axis, on the respective front face. The front faces of the stirring arms 42 are, in other words, vertical so that they theoretically do not lead to any transport of the raw gravel in the longitudinal direction of the container 10 (in the direction of the planes of the front faces). This feature not only leads to an improved working of the raw gravel and to a simpler construction, but also to the advantage that the bearings 46, 48 are not subjected to pull and pushing forces.

The dimensioning of the container 10 is not critical. It is only the distance between the outer ends of the stirring arms 42 and the inner wall of the container and the distance between the inner edges of the annular discs 54 which should be determined in accordance with the maximum diameter of the gravel particle size to be processed or prepared. The distances mentioned are preferably somewhat greater than the diameter of the maximum gravel particles and in any case smaller than twice the diameter.

I claim:

1. In a method for preparing raw gravel for screening or washing of extraneous material therefrom the improvement comprising mechanically comminuting said extraneous lump material by crushing the lump material with stones of the gravel prior to screening or washing, said comminuting step comprising stirring said raw gravel with a stirrer in a vertical container, moving the material from the top to the bottom of the container, and holding said raw gravel in the range of action of said stirrer in said container for a selected time.

2. An apparatus for preparing raw gravel for screening or washing of extraneous material therefrom, said apparatus comprising a tubular container the longitudinal axis of which container is substantially vertical, said container having at least one charging opening at the upper end of said container and at least one discharge opening at the bottom end of said container, the top end and the bottom end of said container comprising end walls for closing the ends of said container, said charging and discharging openings extending through the respective said end walls, stirring means disposed within said container, and means for controlling the charge and discharge of the raw gravel through said charging and discharging openings respectively to keep the container continuously full of said raw gravel in operation thereof.

3. The apparatus of claim 2 wherein the end wall at the charging end of said container has at least two charging openings extending therethrough, and the end wall at the discharge opening of said container has at least two discharge openings extending therethrough, the charging and discharging openings being displaced from the longitudinal axis of said container.

4. The apparatus of claim 2 further comprising a hopper positioned to discharge raw gravel to said charging opening, and means for controlling the speed of discharge of raw gravel through said hopper.

5. The apparatus of claim 4 wherein said means for controlling the speed of discharge of raw gravel comprises rotatable stirring means positioned in said hopper, and motor means for rotating said rotatable stirring means at a controllable speed.

6. The apparatus of claim 2 comprising conveyor trough means positioned to receive raw gravel from said discharge opening for conveying raw gravel away from said container, and motor means for operating said conveyor means at a controllable speed.

7. The apparatus of claim 2 comprising a drive shaft extending through said container, and wherein said stirring means comprises a plurality of stirring arms fixed to said drive shaft, each of said stirring arms having front faces on the side of the respective arms toward which the arm is moved, the portion of the cross section of each arm in a plane parallel to the axis of said shaft defined by the front face thereof extending parallel to the axis of said drive shaft.

8. The apparatus of claim 7 comprising means for controlling the speed of rotation of said drive shaft.

9. The apparatus of claim 2 wherein said stirring means comprises a drive shaft extending through said container, and a plurality of stirring arms affixed to said drive shaft, said stirring arms being arranged in pairs on said drive shaft, with each pair of arms being at the same level thereon and axially displaced from each other pair of stirring arms.

10. The apparatus of claim 9 wherein the stirring arms of adjacent pairs of stirring arms are offset from each other by a constant angle about said shaft.

11. The apparatus of claim 2 comprising sheet metal guide means mounted on the inner wall of said container, said guide means being positioned to force raw gravel passing therethrough into the range of action of said stirring means. 

1. IN A METHOD FOR PREPARING RAW GRAVEL FOR SCREENING OR WASHING OF EXTRANEOUS MATERIAL THEREFROM THE IMPROVEMENT COMPRISING MECHANICALLY COMMINUTING SAID EXTRANEOUS LUMP MATERIAL BY CRUSHING THE LUMP MATERIAL WITH STONES OF THE GRAVEL PRIOR TO SCREENING OR WASHING, SAID COMMINUTING STEP COMPRISING STIRRING SAID RAW GRAVEL WITH A STIRRER IN A VERTICAL CONTAINER, MOVING THE MATERIAL FROM THE TOP TO THE BOTTOM OF THE CONTAINER, AND HOLDING SAID RAW GRAVEL IN THE RANGE OF ACTION OF SAID STIRRER IN SAID CONTAINER FOR A SELECTED TIME
 2. An apparatus for preparing raw gravel for screening or washing of extraneous material therefrom, said apparatus comprising a tubular container the longitudinal axis of which container is substantially vertical, said container having at least one charging opening at the upper end of said container and at least one discharge opening at the bottom end of said container, the top end and the bottom end of said container comprising end walls for closing the ends of said container, said charging and discharging openings extending through the respective said end walls, stirring means disposed within said container, and means for controlling the charge and discharge of the raw gravel through said charging and discharging openings respectively to keep the container continuously full of said raw gravel in operation thereof.
 3. The apparatus of claim 2 wherein the end wall at the charging end of said container has at least two charging openings extending therethrough, and the end wall at the discharge opening of said container has at least two discharge openings extending therethrough, the charging and discharging openings being displaced from the longitudinal axis of said container.
 4. The apparatus of claim 2 further comprising a hopper positioned to discharge raw gravel to said charging opening, and means for controlling the speed of discharge of raw gravel through said hopper.
 5. The apparatus of claim 4 wherein said means for controlling the speed of discharge of raw gravel comprises rotatablE stirring means positioned in said hopper, and motor means for rotating said rotatable stirring means at a controllable speed.
 6. The apparatus of claim 2 comprising conveyor trough means positioned to receive raw gravel from said discharge opening for conveying raw gravel away from said container, and motor means for operating said conveyor means at a controllable speed.
 7. The apparatus of claim 2 comprising a drive shaft extending through said container, and wherein said stirring means comprises a plurality of stirring arms fixed to said drive shaft, each of said stirring arms having front faces on the side of the respective arms toward which the arm is moved, the portion of the cross section of each arm in a plane parallel to the axis of said shaft defined by the front face thereof extending parallel to the axis of said drive shaft.
 8. The apparatus of claim 7 comprising means for controlling the speed of rotation of said drive shaft.
 9. The apparatus of claim 2 wherein said stirring means comprises a drive shaft extending through said container, and a plurality of stirring arms affixed to said drive shaft, said stirring arms being arranged in pairs on said drive shaft, with each pair of arms being at the same level thereon and axially displaced from each other pair of stirring arms.
 10. The apparatus of claim 9 wherein the stirring arms of adjacent pairs of stirring arms are offset from each other by a constant angle about said shaft.
 11. The apparatus of claim 2 comprising sheet metal guide means mounted on the inner wall of said container, said guide means being positioned to force raw gravel passing therethrough into the range of action of said stirring means.
 12. The apparatus of claim 11 wherein said sheet metal guide means are comprised of annular disks mounted within said container, and extending inwardly from the inner wall of said container, said disks having inner diameters less than the outer diameter of range of operation of said stirring means. 